A simple 3rd grade math problem

[Msmith]

Fax has a garage sale. The cost of the sweater is $18. Fax sells it for $21. A person buys the sweater and gives Fax a $100 bill. Fax goes to his neighbor for change. He gives the buyer $79 back. The next day the neighbor tells Fax that the $100 is faked. Fax gives the neighbor back $100. How much money does Fax lose?